1. Field of the Invention
The present invention relates to a method of and apparatus for estimating the characteristics of a radio path or channel over which a reference signal partially defective is transmitted.
2. Description of the Background Art
For the purpose of demodulating information signals transmitted over a transmission or radio channel, the system for estimating the characteristics of the radio channel is adapted to use a reference signal, such as a pilot signal, included in the signals transmitted over the radio channel. There is a type of transmission system using a reference signal that includes a part or field which cannot be directly utilized in estimating the characteristics of the radio channel.
In general, there is a method of estimating the characteristics of a radio channel by continuously watching a pilot channel, which is separately provided from a channel containing user information or utility data signals. With such a method, the pilot signals, which are not modulated with data signals, are consecutively transmitted over a pilot channel separately provided from a utility data channel. A receiver system is adapted to receive the pilot signals consecutively on a time axis and calculate the moving average, for example, of the pilot signals in a predetermined period of time. On the basis of the moving average thus calculated, the receiver in turn estimates the characteristics of the radio channel over which the pilot signals have been transmitted so as to appropriately demodulate the utility data signals.
The cdma2000 system, regulated by the IS-95 Standard, is one of the most interesting services of the mobile telephone, and provides the most attractive features of establishing the much more improved sound quality comparable to the stationary telephone and the much more increased data transmission rate over 64 kbps (bit per second) as well as the global roaming capability. The cdma2000 system introduces the TPC (Transmission Power Control) bits into the pilot channel transmitted on a reverse link from a mobile station to a base station.
The inclusion of the TPC bits in a pilot channel of the cdma2000 system follows that the field of TPC bits in the pilot channel lacks information on the characteristics of a channel for transmitting utility data, giving rise to a difficulty in direct estimation of the channel characteristics.
It is therefore an object of the present invention to provide a method of and apparatus for appropriately estimating the characteristics of a radio channel even when a reference signal transmitted over the radio channel is partially defective.
In accordance with the present invention, the characteristics of a radio channel is estimated over which a reference signal are transmitted consecutively together with an additional signal, other than the reference signal. From the reference signal demodulated into a baseband signal, obtained are a first correlated value of at least a first part of the reference signal which is closer to the additional signal on a time axis and a second correlated value of the additional signal with the spread spectrum codes. The first and second correlated values are operated to determine data represented by the additional signal. The data thus determined are removed from the additional signal to restore a second part of the reference signal which would have appeared unless multiplied with the spread spectrum codes. The characteristics of the radio channel are estimated from the reference signal including the first part and the second part from which the data determined have been removed.
In accordance with the present invention, a method of estimating characteristics of a radio channel over which a reference signal is transmitted consecutively together with an additional signal, other than the reference signal, with a sequence of spread spectrum codes multiplied, comprises: a first step of receiving the reference signal and the additional signal from the radio channel and demodulating the reference signal and the additional signal received into a baseband signal; a second step of obtaining in the baseband signal a correlated value of at least a first part of the reference signal which is closer to the additional signal on a time axis with the sequence of spread spectrum codes to produce a first correlated value; a third step of obtaining in the baseband signal a correlated value of the additional signal with the sequence of spread spectrum codes to produce a second correlated value; a fourth step of operating the first correlated value and the second correlated value to determine data represented by the additional signal; a fifth step of removing the data from the additional signal to restore a second part of the reference signal which would have appeared unless multiplied with the sequence of spread spectrum codes; and a sixth step of estimating the characteristics of the radio channel on the basis of the reference signal including the first part and the second part from which the data have been removed.
Further in accordance with the invention, apparatus for estimating characteristics of a radio channel over which a reference signal is transmitted consecutively together with an additional signal, other than the reference signal, with a sequence of spread spectrum codes multiplied, comprises: a demodulator for receiving the reference signal and the additional signal from the transmission channel and demodulating the reference signal and the additional signal received into a baseband signal; a first correlator circuit for obtaining in the baseband signal a correlated value of at least a first part of the reference signal which is closer to the additional signal on a time axis with the sequence of spread spectrum codes to produce a first correlated value; a second correlator circuit for obtaining in the baseband signal a correlated value of the additional signal with the sequence of spread spectrum codes to produce a second correlated value; a determining circuit for operating the first correlated value and the second correlated value to determine data represented by the additional signal; a data remover circuit for removing the data from the additional signal to restore a second part of the reference signal which would have appeared unless multiplied with the sequence of spread spectrum codes; and an estimator circuit for estimating the characteristics of the radio channel on the basis of the reference signal including the first part and the second part with the data removed by said data remover circuit.
There is also provided in accordance with the invention, apparatus for demodulating a utility data signal transmitted over a radio channel, over which a reference signal is transmitted consecutively together with an additional signal, other than the reference signal, with a sequence of spread spectrum codes multiplied, which comprises: a demodulator for receiving the reference signal and the additional signal from the radio channel and demodulating the reference signal and the additional signal received into a baseband signal; a first correlator circuit operative in response to an estimate signal representative of estimated characteristics of the radio channel for correlating the utility data signal of the baseband signal with the sequence of spread spectrum coded to develop a resultant utility data signal; a second correlator circuit for obtaining in the baseband signal a correlated value of at least a first part of the reference signal which is closer to the additional signal on a time axis with the sequence of spread spectrum codes to produce a first correlated value; a third correlator circuit for obtaining in the baseband signal a correlated value of the additional signal with the sequence of spread spectrum codes to produce a second correlated value; a determining circuit for operating the first correlated value and the second correlated value to determine data represented by the additional signal; a data remover circuit for removing the data from the additional signal to restore a second part of the reference signal which would have appeared unless multiplied with the sequence of spread spectrum codes; and an estimator circuit for estimating the characteristics of the radio channel on the basis of the reference signal including the first part and the second part with the data removed by said data remover circuit to deliver the estimate signal representing the characteristics estimated to said first correlator circuit.